Male-Killing Bacteria in Insects:
Mechanisms, Incidence, and Implications
Gregory D.D. Hurst and Francis M. Jiggins
University College London, United Kingdom Bacteria that are vertically transmitted through female hosts and kill male hosts thatinherit them were first recorded in insects during the 1950s. Recent studies have shownthese “male-killers” to be diverse and have led to a reappraisal of the biology of manygroups of bacteria. Rickettsia, for instance, have been regarded as human pathogenstransmitted by arthropods. The finding of a male-killing Rickettsia obligately associatedwith an insect suggests that the genus’ members may be primarily associated witharthropods and are only sometimes pathogens of vertebrates. We examined both howkilling of male hosts affects the dynamics of inherited bacteria and how male-killingbacteria affect their host populations. Finally, we assessed the potential use of thesemicroorganisms in the control of insect populations.
investment in daughters at the expense of sons.
bacteria they pass on to their progeny. These In these cases, particular host lines produce inherited bacteria are often beneficial symbionts female-biased sex ratios, a trait that is inherited that play a key role in host metabolism. In many but curable with antibiotics. We considered one cases (e.g., the aphid symbiont Buchnera), the class of these, the male-killing bacteria, in which bacteria are maintained in a special host organ, infection of a female results in the production of the bacteriome, with the host controlling female-biased broods because male progeny die transmission to progeny, and show evidence of cospeciation (1,2). In these cases, destroying thebacteria (e.g., through antibiotic treatment) Systematics of Male-Killing Bacteria
causes a profound loss of host performance. In Molecular systematic approaches have shown other cases, inherited bacteria are not integrated that male-killing bacteria derive from many into host physiology and anatomy and do not different clades. In most cases, the data come show long-lived relationships with their host, as from DNA sequencing of bacteria associated with indicated by a lack of cospeciation (3). These the trait and confirmation of the trait association bacteria may be broadly separated into two by polymerase chain reaction across infected and classes. First, bacteria maintained through a uninfected lines. Because inherited microorgan- phase of horizontal transmission (e.g., Rickettsia isms are difficult to culture, Koch’s postulates prowazekii), with transmission to other arthro- have been fulfilled formally in only two cases pod hosts often occurring through a vertebrate or (4,5). Given this caveat, male-killing bacteria plant intermediate host (infection of the have been found within the genus Spiroplasma (Mollicutes) (4,6), the Flavobacteria-Bacteroides infection follow from host feeding); second, group (7), and the gamma and alpha subdivisions bacteria that rarely show horizontal transmis- of the proteobacteria (5,8,9) (Figure).
sion, but are maintained because they manipu- Male-killing bacteria derive from arthropod- late host reproduction. One set of manipulations associated bacterial clades that are not them- manifested by these bacteria is increasing selves male-killers. The clades can be separatedinto two types according to the transmissionmechanisms of bacteria within them: first, Address for correspondence: Department of Biology, UniversityCollege London, 4 Stephenson Way, London NW1 2HE, U.K.; entirely horizontal transmission or a mix of Fax: +44 20 73832048; e-mail: [email protected].
horizontal and vertical transmission; and second, Perspectives
Figure. Phylogenetic re-lationship of male-kill-ers and a selection ofother eubacteria in-ferred from 16S rDNAsequences, using maxi-mum likelihood imple-mented on PAUP*. Themale-killing bacteria(underlined) have beenlabeled with the nameof their insect host if aspecies name is notavailable. The relation-ships of the major bac-terial groups are uncer-tain.
horizontal transmission that is not epidemiologi- established. The fact that male-killers derive cally important. In the first type of clade are the from such groups suggests this possibility.
genera Spiroplasma and Rickettsia. In Spiroplasma, most members have either hori- transmission rates far exceed those of horizontal zontal transmission only (after feeding on a plant transmission. Wolbachia and the flavobacterial host) or a mix of horizontal and vertical lineage associated with arthropods are in this transmission between arthropod hosts (10).
group. Wolbachia are usually maintained Rickettsia most commonly have a mix of through manipulation of their host’s reproduc- horizontal and vertical transmission, with tion (12). The closest relative of the flavobacterial horizontal transmission occurring after feeding male-killer is Blattabacterium, the beneficial on a vertebrate host. As recently as 10 years ago, inherited bacterium of cockroaches and ter- Rickettsia was regarded as one of the vertebrate Male-killing, a trait that evolves in bacteria Rickettsia that show transmission after feeding already maternally inherited in arthropods, can on plant hosts are increasingly being recognized occur if the ancestral agent is obligately vertically (11), and the finding of a male-killing Rickettsia transmitted or a mix of vertical and horizontal in ladybird beetles (8) suggests that the group is transmission is present. Therefore, male-killing associated with arthropods, some members of strains are likely to be common in the genus which cause disease in vertebrates. Other male- Spiroplasma and the alpha group of killing strains of Rickettsia will most likely be proteobacteria. Furthermore, the diversity of found. However, whether a bacterium from these agents suggests that there is no taxonomic bar to groups could evolve male-killing yet retain where the transition to male-killing can take horizontal transmission between females via place. Thus male-killing strains are also likely to feeding on a plant or vertebrate host has not been be found in the spirochetes and perhaps the beta Perspectives
and delta divisions of the proteobacteria, as these groups are known to be vertically transmitted coccinellid (ladybird) beetles. Approximately half of aphidophagous species bear male-killers, and Although vertical transmission of male-killing one species (Adalia bipunctata) is host to at least bacteria is the rule, transmission between host three male-killing bacteria (6,8,9).
species has occurred. In Spiroplasma, the relatives of S. ixodetis cause male-killing in distantly only in insects. However, the range of insect hosts related hosts (a butterfly and a ladybird beetle) (6,14). The evolutionary distance between beetles determination systems. Given that close rela- and butterflies indicates that the bacteria do tives of male-killing bacteria are found in cross between host species over evolutionary time.
noninsect arthropods (e.g., Spiroplasma andRickettsia in ticks) and the conditions for the Host Species Affected
spread of male-killing strains are met outside The incidence of male-killing bacteria varies insect hosts, cases of male-killing are likely to with host ecology and biology. The trait of male- occur in species other than insects. Two examples killing is adaptive when the death of males merit particular examination. First, infection promotes the survival of female siblings. If the with Orientia tsutsugamushi is associated with bacteria can be transmitted only vertically, the production of all-female broods in the trombiculid death of male hosts can at worst be neutral (i.e., mite, Leptotrombidium fletcheri (18,19); in this they cannot transmit the bacterium). Death of example, the nature of the resultant sex-ratio males is adaptive if it increases the survival of distortion (primary vs. secondary bias) needs to sibling females, who bear the same bacterium by be assessed. Second, in the case of Spiroplasma ixodetis and its tick host Ixodes pacificus, the The features of host biology and ecology that association of closely related bacteria with male- increase the benefit to the bacterium of killing killing in insects needs to be assessed.
male embryos are sibling egg consumption(females eat their dead brothers), antagonistic Prevalence of Male-Killers
interactions between siblings (male-killing may in Natural Populations
reduce both cannibalism of females and the The prevalence of male-killers in natural intensity of competition between siblings), and populations varies with host species (Table 1). A deleterious inbreeding (15-17). These observa- prevalence value of 5%-50% might be “normal” tions explain why male-killer hosts commonly lay among female hosts; however, in some cases eggs in clutches. Incidence is highest where there prevalence is very low (e.g., 1% in Drosophila Table 1. Prevalence of male-killers in natural populations of their insect hosts (proportion of females infected) Perspectives
willistoni [20]), and in some exceptional species > causes death in males only). What we know 90% of females are infected (e.g., the butterfly derives almost exclusively from study of the Acraea encedana [21]). However, there is likely to interaction between Spiroplasma poulsonii with be study bias towards high-prevalence infections, and all very low-prevalence infections occur in Studies of embryos from D. willistoni lines drosophilids, where large samples can easily be infected with S. poulsonii show that death occurs bred. Infection prevalence also commonly varies at two stages (23): 1) before gastrulation, between populations within a host, and preva- associated with abnormal cleavage patterns; in lence can vary on a remarkably small scale. In the particular, achromatic spindles, with other walnut leaf beetle (Gastrolina depressa) in abnormalities of the mitotic process, which Japan, male-killers are absent in populations at account for most embryonic deaths in male-killed the north and south of the islands but present in lines. 2) After gastrulation, not associated with 50%-80% of females in the center of the islands the normal brown coloration of necrotic embryos; (22). Prevalence variation on a kilometer scale rather, the embryo blackens as a result of exists in Acraea encedon (21).
breakdown of internal structures and pycnosis of Prevalence is determined by the physiologic effect of infection on female host performance, The points of interaction between host and the transmission efficiency of the bacterium from bacterium have been investigated in D. melanogaster mother to progeny, and the level of advantage to lines transfected with S. poulsonii. In Drosophila, male-killing (determined by host factors such as sibling egg consumption) (Table 2). Transmission chromosomes to autosomes. In females, which efficiency may be influenced by the environment are 2X:2n, the peptide Sxl is produced. Sxl (e.g., high temperatures may lower transmission induces female development of the soma and the efficiency), the bacterium, and the host. Selection germ line. In males, which are X: 2n, Sxl is not favors host genes that impede the transmission of produced. Absence of Sxl is associated with the bacteria from mother to progeny. The spread upregulation of genes on the single X chromo- of host resistance genes may prevent infections from commonly reaching the high prevalence development, and male germ line development.
achieved by other inherited bacteria.
In Drosophila, the male-killer does not interactwith any part of the somatic sex development Table 2. Factors affecting the prevalence of male-killing pathway. Mutants of the tra gene bear two X chromosomes and produce Sxl but develop as Increase
somatic males. They are not, however, killed by Decreased rate of inbreeding suffered by female hosts S. poulsonii (24). Thus, the interaction between Increased access to early resources through male-killer and host is not associated with somatic sex, so the target of detection and Increased access to resources due to reduced virulence is either before Sxl is produced, Sxl competition, following death of sibling male hosts itself, or the dosage compensation or germ-line Decrease
Although the interaction between Drosophila Inefficiency in vertical transmissionDirect physiologic costs of infection and S. poulsonii is the only one studied in any detail, it appears that the mechanism of sex determination exhibited by different male-killerhosts varies widely. Male-killing bacteria havebeen observed in male heterogametic, femaleheterogametic, and haplodiploid hosts. Fur- Mechanism of Male-Killing
thermore, members of the same clade of male- Little is known about how male-killing is killers can be found in hosts of different sex achieved. Neither the cue used to detect sex nor determination systems. The same Spiroplasma the mechanism by which death is brought about kills males in ladybirds (male heterogametic) and is known in any detail. Indeed, rather than two butterflies (male homogametic). Similarly, steps (detection then virulence) there may be male-killing Wolbachia have been observed in only one (constitutive production of a factor that both male and female heterogametic species Perspectives
(9). Given that male and female heterogametic Empiric studies have suggested that infec- tion usually decreases the performance of female directions and show different patterns of hosts (25,26). The one exception is the interaction dosage compensation, the fact that male-killers between Spiroplasma poulsonii and members of operate in both these hosts suggests that the the Drosophila willistoni group, in which larval development is accelerated by infection (27,28).
dosage compensation pathway may not be the However, infection is also associated with focus of male-killing activity; rather, somatic increased sterility and decreased longevity sex determination or germ-line sex determina- among adult females (28). Male-killing bacteria, unlike beneficial symbionts, are spread through- Experiments with S. poulsonii demonstrate out host tissues, and the bacteria may be present that the somatic sex determination system is not in very high numbers. Drosophila are infected the focus of male-killing behavior. In the case of with extremely high titers of S. poulsonii within the other male-killing Spiroplasma, the presence the hemolymph (29). Adalia bipunctata hemocytes of the bacterium in species of different sex are regularly infected with Rickettsia (30).
determination systems suggests that the focus is Beneficial effects of male-killing bacteria on either the somatic sex determination or the germ- host performance cannot yet be ruled out.
line determination system. Two conclusions are However, positive effects may be fewer than therefore possible: germ-line determination is those found in the “classical” beneficial agents, the focus of male-killing in all cases, or male- which typically perform a vital metabolic killing has more than one basic mechanism.
function that insects are unable to perform. Male- Further research is clearly warranted.
killers infect a minority of females and are rarelycarried by larval or adult males. Thus, although Direct Effects on Female Hosts
they may add to host performance, they cannot The interaction between male-killing bacte- substitute for any part of it. A host cannot be ria and their female hosts is interesting. On the dependent on a male-killer for a physiologic one hand, there is selection for a reduction in the function as it can on a beneficial symbiont.
number of bacteria present in the host(minimizing virulence) and for a direct Population and Evolutionary
physiologic contribution to host metabolism.
Effects on Hosts
Invasion of a host population by male-killing associated with the fidelity of their transmis- bacteria affects the dynamics of the host sion to progeny. There may be a trade-off population and alters the pattern of selection on between minimizing virulence and maximizing the population to ameliorate the effects of the vertical transmission efficiency, especially if parasite (Table 3). A high prevalence of male- such efficiency is positively related to bacterial killers may increase the proportion of female number. Thus these bacteria can be either hosts that fail to mate (31), potentially reducing detrimental (if the density of bacteria is high to the population size of the host. A dearth of males ensure vertical transmission) or beneficial to the can subtly alter the mating system of the host.
host (if the bacteria play a role in host Choice by females of male mates and competition among males for mating opportunities are the Table 3. Population and evolutionary effects of invasion of a host by male-killing bacteria Reduced population density at larval level due to death of male embryosFailure of females to find mates where parasite Selection for genes that prevent transmission prevalence leads to shortage of males, with potential effects on adult population sizeAltered epidemiology of sexually transmitted Alteration in host pattern of sexual selection due to pathogens due to increased reproductive success Perspectives
rule in insects. However, the biased population to reduce host population size. Alternatively, sex ratios that result from the spread of male- killing bacteria can reverse this pattern (31).
schemes based on release of sterile males, so that Male choice of females and competition among they may amplify the effect of sterile releases on females for males is expected, with a relaxation of the population size of adult males. In addition, selection on males to ensure paternity.
the recent discovery of male-killing in the clade Male-killers that have invaded populations Wolbachia adds an extra dimension to the use of may cause changes to host biology. Theory predicts this organism in direct and transgenic control of selection for an increase in the size of clutch produced (32). Most importantly, genes that The usefulness of male-killers in reducing prevent the action or transmission of the parasite pest damage on their own is debatable. Insect will be favored. The presence of these genes has population size and population persistence are been reported (33), but their nature and mode of largely a function of female, not male, number.
action are unknown. The means by which insects Thus, although the presence of a male-killer may exclude bacteria is clearly of great import in our reduce larval density, it is unlikely to decrease understanding of insect-borne diseases, and the the population size of breeding females.
nature of resistance genes is expected to be an Furthermore, the presence of density depen- dence during the larval stages is likely to reduce One of the issues to be determined relates to the effect of male death on numbers of larvae.
whether male-killing bacteria can cause the Perhaps a more realistic use of male-killing extinction of their host. The case of the butterflies Acraea encedon and A. encedana is suggestive.
conjunction with sterile male release systems of The Wolbachia male-killer in these species is at control. In sterile male release, control is high prevalence and clearly has some impact on achieved through release into the environment of the host population (21,31). If a male-killing mass-produced sterile males, which mate with bacterium showed perfect vertical transmission, females and lower their fertility (34). The success host extinction would be likely. However, of sterile male release depends on maintaining a selection on the host acts to lower bacterial high ratio of sterile to normal males in the transmission efficiency, which may ultimately population. The presence of a male-killer in the host population lowers the number of fertilemales and thus increases the effectiveness of any Conclusions: Implications and Uses
release. The effects of male-killing bacteria at of Male-Killing Bacteria
different prevalences on sterile male release, in Male-killing is an adaptive trait that aids the conjunction with the effects on host population spread of inherited bacteria through natural dynamics, need to be investigated. However, populations. The presence of male-killing strains direct use of male-killing bacteria as an aid to in many bacterial taxa clearly indicates that controlling host numbers is only achievable as a male-killing should be considered in epidemio- long-term stratagem. Following release of logic investigations of vertically transmitted infected hosts into natural populations, spread bacteria. Male-killing is perhaps most important will occur only in hosts with suitable ecologies in interactions between arthropods and Rickett- and significant prevalence levels will be achieved sia and Spiroplasma. Members of these genera over a period of years rather than weeks.
frequently show horizontal transmission be- Another potential application of male-killing tween arthropod hosts (after host-feeding), as bacteria in the sphere of pest and disease vector well as vertical transmission in the arthropod control may occur indirectly through study of the host. Given that some bacteria in these groups virulence mechanisms of male-killers.
induce male-killing, testing for the presence orabsence of this trait should be a part of future Acknowledgments
investigations of their epidemiology.
The authors thank Andrew Pomiankowski and two anonymous reviewers for their comments on the manuscript.
bacteria in pest control has yet to be properly In conducting this study, Greg Hurst was supported by assessed. Male-killers may be used on their own a BBSRC D Phillips Fellowship and Frank Jiggins by aBBSRC studentship.
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